1. Field of the Invention
The present disclosure relates to a graphene paper which graphene oxide layers and coating layers are stacked in sequence and a preparation method thereof.
2. Description of the Related Art
Generally, graphite is a stack of two-dimensional graphene sheets formed from a planar array of carbon atoms bonded into hexagonal structures. Recently, when single or multiple graphene layers were removed to analyze the properties and found useful properties of graphene sheets that were different from existing material.
The most noticeable of these properties is that electrons flow as zero mass in the graphene sheets. As a result, the electrons flow at the velocity of light in a vacuum, which means flowing in the speed of light. In addition, an anomalous half-integer quantum hall effect for both electron and electron holes are observed in graphene sheets.
Currently, the electron mobility of graphene sheet is as high as about 20,000 to 50,000 cm2/Vs. Also, it is preferable to manufacture products using graphene sheets rather than carbon nanotubes, since products made from graphite are inexpensive while products made from carbon nanotubes which are used in applications similar to those in which graphene sheets are used, are expensive due to low yields during synthesis and purification processes even though the carbon nanotubes themselves are inexpensive. Single wall carbon nanotubes exhibit different metallic and semiconducting characteristics according to their chirality and diameter. Furthermore, single wall carbon nanotubes having identical semiconducting characteristics have different energy band gaps depending on their chirality and diameter. Thus, in order to obtain a metallic single wall carbon nanotube composition or a semiconducting single wall carbon nanotube composition, it is desirable to separate the single wall carbon nanotubes from each other in order to obtain desired metallic or semiconducting characteristics respectively. However, separating single wall carbon nanotubes is not a simple or inexpensive process.
On the other hand, it is advantageous to use graphene sheets since it is possible to design a device to exhibit desired electrical characteristics by arranging the graphene sheets so that their crystalline orientation is in a desired direction since their electrical vary with crystalline orientation. It is envisioned that these characteristics of graphene sheets will render them useful in carbonaceous electrical devices or carbonaceous electromagnetic devices in the future.
However, although graphene sheets have these advantageous characteristics, a process of economically and reproducibly preparing a large-sized graphene sheet has not yet been developed.
Three methods that are currently available for preparing graphene are micromechanical method, SiC crystal thermal decomposition method and chemical vapor deposition method (CVD).
According to the micromechanical method, a graphene sheet separated from graphite can be deposited on the surface of Scotch™ tape by attaching the tape to a graphite sample and detaching the tape. In this case, the separated graphene sheet does not include a uniform number of layers, and the graphene sheets have an irregular shape similar as a ripped paper. Furthermore, a large-sized graphene sheet cannot be prepared using the micromechanical method.
In SiC crystal thermal decomposition method, a single crystal SiC is heated to remove Si by decomposition of the SiC on the surface thereof, and then residual carbon C forms a graphene sheet. However, single crystal SiC which is used as the starting material is very expensive, and a large-sized graphene sheet cannot be easily prepared.
In addition, in chemical vapor deposition method, transition metals such as nickel (Ni) or copper (Cu) are used as the catalyst layer which can adsorb carbon at high temperatures. When reacted with methane and hydrogen gas mixture, the carbon atom adsorb to the catalyst layer. These carbon atoms contained in the catalyst layer are crystallized on the surface when cooled, therefore generating graphene.
However, chemical vapor deposition method is advantageous for obtaining large-sized graphene with identical crystalline forms, but graphene can not exist in independent form since they are always transcribed on the substrate surface.
Meanwhile, an attempt has been made to prepare graphene using a chemical process. However, the process is not completely controllable. Another method of preparing graphene was by forming and dispersing graphene oxide. Since graphene oxide is easily dispersed, a thin layer made of the graphene oxide can be easily formed. Other attempts are being made to manufacture graphene by reducing the graphene oxides.
Patent Document 1 [Korean Patent Application No. 10-2007-0126947] discloses a method of preparing a reduced graphene oxide thin layer comprising the steps of: forming a graphene oxide layer by coating a graphene oxide dispersion on a substrate; reducing the graphene oxide by immersing the substrate comprising the graphene oxide layer in a reducing agent-containing solution; and doping an organic dopant and/or inorganic dopant on the reduced graphene oxide.
Patent Document 2 [Korean Patent Application No. 10-2009-0013137] discloses a graphene sheet including intercalation compound and a preparation method thereof, to control the electrical, optical, and physical properties of the graphene sheet according to the intercalated compounds without deteriorating the intrinsic electrical properties, transmittance, and flexibility of the graphene sheet.
Patent Document 3 [Korean Patent Application No. 10-2009-0054708] discloses a method of manufacturing a large-sized graphene film through the process of spin drying and reducing a graphene film dispersed in a hydrophilic solution, comprising the steps of: manufacturing a dispersion solution by dispersing graphene oxide in a hydrophilic solution; spin drying the dispersion solution; and reducing the graphene oxide film obtained by spin drying.
However, none of the above disclosed methods could perfectly reproduce the mechanical properties and electrical properties of the graphene.
Thus, the present inventors have performed intensive research to develop a graphene paper with excellent electrical conductivity and mechanical properties. As a result, the present inventors identified that graphene paper which the reduced graphene oxide layers and coating layers are stacked in sequence prepared according to the method of the present invention has excellent electrical conductivity and mechanical properties, thereby leading to completion of the present invention.